A large number of methods for producing an N-vinylcarboxylic acid amide have hitherto been proposed. For example, a method is known where an N-(1-alkoxyethyl)-carboxylic acid amide as an intermediate is produced from a carboxylic acid amide, acetaldehyde and an alcohol, and an N-vinylcarboxylic acid amide is synthesized by cracking or catalytic cracking of the product. Another useful method for synthesizing an N-vinylcarboxylic acid amide includes synthesizing an ethylidene biscarboxylic acid amide from acetaldehyde and a carboxylic acid amide. This product is then cracked into a carboxylic acid amide and an N-vinylcarboxylic acid amide.
In these methods, a purification step, such as distillation, extraction and recrystallization of the resulting N-vinylcarboxylic acid amide is provided. For example, JP-A-61-286069 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses an extraction separation using water and an aromatic hydrocarbon. This is because distillation fails to prevent mingling of formamide as an unreacted raw material into the N-vinylformamide. Furthermore, JP-A-63-132868 discloses cooling crystallization from a mixed organic solvent, JP-A-2-188560 discloses extraction using an aqueous inorganic salt solution and an aromatic hydrocarbon, and U.S. Pat. No. 4,401,516 discloses extractive distillation using a polyhydric alcohol.
On the other hand, an N-vinylcarboxylic acid amide polymer is obtained by polymerizing an N-vinylcarboxylic acid amide alone or copolymerizing with another monomer. The polymer is used as a coagulant, a liquid absorbent or a thickener, and in any case the polymer must have a high molecular weight. However, it has been difficult to obtain an N-vinylcarboxylic acid amide which consistently exhibits good polymerizability in any of the uses described above.